1. Field of the Invention
This invention relates to the production of dense, alumina-based ceramic materials which are useful as, among other things, abrasive grains. In one aspect, this invention relates to producing dense alumina-based ceramics from a sol-gel process while in another aspect, the invention relates to the use of densification aids in the process which facilitate sintering, and thereby improves the densifications of the final product.
2. Description of the Prior Art
The preparation by a sol-gel process of dense, alumina-based ceramic abrasive grain is known. U.S. Pat. No. 4,314,827 describes a process of making an abrasive mineral employing chemical ceramic technology by gelling a colloidal dispersion or hydrosol of alumina containing at least one precursor of at least one modifying component followed by dehydration and firing. A preferred method of addition of the modifying component is in the form of a soluble salt such as magnesium nitrate. Other disclosures in this area include U.S. Pat. No. 4,181,532 and "Application of Sol-Gel Processes to Industrial Oxides", Jan. 13, 1968, Chemistry and Industry.
While all of these references disclose useful techniques for making dense alumina based ceramic material useful as abrasive grains, none produces a completely satisfactory product. The material which results from most, if not all, of these processes is well below its theoretical density. However, improvement with respect to the density achieved under conventional firing conditions can be made. When used as abrasives, this increased density results in an improvement to their performance in abrasive products. Moreover, most of the sol-gel processes described in these references can be improved in terms of the source of alumina monohydrates which they process into an acceptable product. Typically acceptable results are achieved when expensive, very high quality alumina monohydrates are used while the use of less expensive materials may require special processing steps.
The use of various additives to control porosity in alumina-based ceramics produced by a sol-gel process is also known. U.S. Pat. No. 3,417,028 describes a process in which a water-dispersible polyethylene glycol may be incorporated into a hydrous gel of alumina. The mixture is calcined to remove the glycol completely resulting in an inorganic product of increased pore volume and pore size which is useful as a catalyst carrier. Other disclosures in the art include U.S. Pat. No. 2,816,079 (ethylene oxide) and U.S. Pat. No. 3,325,247 (polyethylene glycol). While these references describe useful porous alumina-based ceramics, tney do not suggest that improvement in terms of increased density or reduced open porosity of alumina monohydrate based sol-gel process alumina ceramics may be obtained when sintered at high temperature from the use of these organic additives.